Stroke write symbol generator

ABSTRACT

A stroke generator in a CRT display uses two integrators to generate straight lines for X and Y deflections respectively, and uses the same two integrators connected by simple switches as a harmonic oscillator to generate circular arcs.

United States Patent Waehner [451 Nov. 21, 1972 [54] STROKE WRITE SYMBOL GENERATOR [72] Inventor: Glenn C. Waehner, Scarsdale, NY.

[73] Assignee: United Aircraft Corporation, East Hartford, Conn.

[221 Filed: Dec. 24, 1970 211 Appl.No.: 101,236

[52] U.S. Cl ..315/24, 315/23 [51] Int. Cl .1101} 29/70 [58] Field of Search ..3l5/23, 24, 27

[56] References Cited UNITED STATES PATENTS 3,388,285 6/1968 Christopher et a] ..3l5/27 11/1968 Henderson et al ..3l5/24 OTHER PUBLICATIONS Pulse, Digital, and Switching Waveforms -Millman & Taub, 1965, page 17.

Primary ExaminerCarl D. Quarforth Assistant ExaminerJ. M. Potenza Attorney-Melvin Pearson Williams [5 7] ABSTRACT A stroke generator in a CRT display uses two integrators to generate straight lines for X and Y deflections respectively, and uses the same two integrators connected by simple switches as a harmonic oscillator to generate circular arcs.

1 Claim, 1 Drawing Figure STROKE WRITE SYMBOL GENERATOR BACKGROUND OF THE INVENTION 1. Field of Invention This invention relates to cathode ray tube deflection circuitry, and more particularly to circuitry for generating symbols on a stroke written display.

2. Description of the Prior Art.

One of the techniques to generate characters on a CRT display employs a stored matrix such as a monoscope. It has the advantage of requiring only a single computer command to generate a complete character. This technique limits the display of characters to those of a fixed size; the number of characters is limited to the characters stored in the matrix. A more versatile technique for generating characters is a stroke write generator. One stroke write technique is to trace out a fixed pattern repeatedly on a CRT screen and selectively blank out the unwanted sections. Even if only a small segment of the fixed pattern is required, the entire pattern is traced out for every character. Naturally, the character size is limited to the pattern size and character time is the time required to trace out the entire pattern. A very different method of character generation than those hereinbefore mentioned is required to generate characters of almost any size or complexity.

A more versatile type of symbol generator makes symbols out of straight line vectors called strokes. Thus, a curved line is made out of short strokes just as a circle is approximated by a polygon. Since a separate computer command is required for each straight line segment, this system requires a relatively large computer memory to produce characters at a reasonable quality level. Other techniques of stroke writing employ a stroke generator which has separate curvilinear mode and straight line mode. An example of such a system is shown in Dye US. Pat. No. 3,394,367. These stroke generators cannot produce high character quality because of transients and discontinuities that occur due to the mode switching operation.

Of the systems that have been devised for use as stroke write symbol generators, either the writing time is slow, character size and quality is limited or they require a relatively large memory and in general are complex and expensive.

SUMMARY OF INVENTION The object of the present invention is to provide a simple and improved stroke write symbol generator which generates vectors at any angle or magnitude as well as smooth curves.

According to the invention, two integrators and an amplifier connected in a feedback loop as a harmonic oscillator can generate signals that result in curvilinear deflection on a CRT display, and switching means, opening the feedback loop, conditions the symbol generator for generating straight line segments.

The present invention generates characters more smoothly than heretofore possible and simplifies character generation so that it can be implemented with fewer components than character generators heretofore known to the art.

Other objects, featuresand advantages of the present invention will become more apparent in the light of the following detailed description of a preferred embodiment thereof as illustrated in the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE herein is a schematic block diagram of a preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing, a stroke write symbol generator is composed of an integrator 10 connected by a switch 12 to the input of a second integrator 14. The output of the second integrator 14 is connected to the input of an amplifier 16. The output of the amplifier 16 is connected by a switch 18 to the input of the integrator 10. The switches 12, 18 are always operated together; that is, both are open, or both are closed.

The output of the integrator 10 provides the X deflection signal on a line 15 for a CRT display 17 a the output of the second integrator 14 provides the Y deflection signal on a line 19. The integrator 10 is composed of an operational amplifier 20 whose non inverting input is connected to a suitable potential reference, such as ground 21. A capacitor 22 is connected from the output of the operational amplifier 20 to its inverting input. A resistor 24 connects the integrator input 23 to the inverting input of the operational amplifier 20. A voltage is provided by a signal source 26 which is connected through a resistor 27 to the inverting input of the operational amplifier 20. A second integrator 14 is similar to the integrator 10 and is driven by the integrator 10 through the switch 12. A voltage is provided by a signal source 28 which is connected to a second input 29 of the integrator 14. The amplifier 16 has a gain of minus 1 and receives its input from the output of the integrator 14.

When the switches 12, 18 are closed, the stroke write symbol generator is in the well known configuration of a harmonic oscillator and operating in a mode that will generate smooth curvilinear arc. The sweep speed (in radians per second) is equal to the reciprocal of the resistor-capacitor product of the resistor 24 and the capacitor 22. The product of resistor 24 and capacitor 22 should be the same at both integrators l0, l4. Initially, a circle will be generated, with the X and Y coordinate of the starting point on the circumference determined by the output of the integrators l0, 14 immediately prior to the closure of the switches 12, 18. The X coordinate of the center of the circle is proportional to the negative of the voltage provided by the signal source 28. The Y coordinate is proportional to the voltage provided by the signal source 26.

Upon opening the switches 12, 18 the signal generator operates in mode that will generate straight line segments. The X deflection voltage is the negative of the integral of the voltage provided by the signal source 26 and appears at the output of the integrator 10. The Y deflection voltage is the negative of the integral of the voltage provided by the signal source 28 and appears at the output of the second integrator 14. Since the sweep time is for a fixed period, the slope and magnitude of the composite deflection is determined solely by the magnitude of the voltages provided by the signal sources 26, 28. The starting point of a straight line segment is determined by the outputs of the integrators 10, 14 immediately prior to opening the switches.

An essential feature of the invention is that by changing the voltages provided by the signal sources 26, 28 during a sweep when the switches l2, 18 are closed, smooth curves other than circles can be generated. Similarly, when the switches 12, 18 are open, a change in either or both of the voltages provided by the signal sources 26, 28 during a sweep will cause a change in slope of deflection during the sweep.

Another essential feature of the invention is that switching cannot cause transients or discontinuities of the display because the termination of one mode (lines or arcs) is the initial condition for the start of the other mode (arcs or lines). Since the elements generating the display are integrators, which inherently resist rapid change of their outputs, they naturally smooth out transients and thereby further contribute to smooth, appealing character generation.

One of the many useful applications contemplated of the invention is where the switches 12, 18 and signal sources 26, 28 are computer controlled. In this application the signal sources 26, 28 may each be a digital to analog converter driven by digital words supplied by the computer. The switches 12, 18 may conveniently comprise computer controlled solid state switches, such as transistors, as contemplated in the aforementioned Dye patent. Separation of characters on the display is achieved by blanking the display during computer controlled intervals.

Although the invention has been shown and described with respect to a preferred embodiment thereof, it should be understood by those skilled in the art that various changes and omissions in the form and detail thereof may be made therein without departing from the spirit and the scope of the invention.

Having thus described a typical embodiment of my invention, that which I claim as new and desire to secure by Letters Patent of the United States is:

1. Stroke write symbol generating means comprising:

means for generating a waveform including a harmonic oscillator composed of a first integrator to generate X deflection voltages, a second integrator to generate Y deflection voltages, and an amplifier with a gain of I, all connected in series into a closed loop;

means applying a respective input voltage signal to each of said integrators; and

switching means operable to isolate the input of each integrator from the output of the other integrator, the means for generating providing a linear waveform having a slope associated with the values of said input voltage signals in response to said integrators being isolated and providing a curvilinear waveform having a center of curvature associated with said input voltage signals in response to said integrators being connected. 

1. Stroke write symbol generating means comprising: means for generating a waveform including a harmonic oscillator composed of a first integrator to generate X deflection voltages, a second integrator to generate Y deflection voltages, and an amplifier with a gain of -1, all connected in series into a closed loop; means applying a respective input voltage signal to each of said integrators; and switching means operable to isolate the input of each integrator from the output of the other integrator, the means for generating providing a linear waveform having a slope associated with the values of said input voltage signals in response to said integrators being isolated and providing a curvilinear waveform having a center of curvature associated with said input voltage signals in response to said integrators being connected. 